The present invention relates to a method for coating the face of a roll in a film size press with a coating agent by a coating device in which a coating doctor extends across the machine width. The coating agent is introduced into the coating device before the coating doctor in the direction of rotation of the roll, as a film onto the roll face. The coating doctor is loaded against the face of the roll in the film size Dress so that the coating doctor spreads and smooths the coating agent on the roll.
The present invention also relates to a device for coating the face of a roll in a film size press by means of a coating device having a coating doctor which extends across the machine width. The coating agent is directed from the coating device against the roll before the coating doctor in the direction of rotation of the roll. The coating doctor is loaded against the face of the roll in the film size press so that the coating doctor spreads and smoothes the coating agent introduced from the coating device as a film onto the roll face.
Currently, in surface sizing or pigment coating techniques for paper or board, a coating blade, a grooved small-diameter bar having a diameter of about 10 mm, or a smooth or grooved large-diameter bar having a diameter in a range from about 20 mm to about 50 mm, is used as coating doctors for the roll face in a film size press. It is a disadvantage that small-scale streak formation arises in the film that is formed on the face of the roll in the film size press especially with high running speeds and with high dry solids contents in the coating paste, in particular in film size presses wherein a smooth bar is used. The small-scale streak formation in the film face may also be characterized as cavitation streaks.
In a film size press wherein a coating blade is used, a deposit of coating agent tends to gather at the tip of the blade. This deposit, when it dries, draws streaks in the coating film, and may be broken as particles onto the face of the coating film. The broken particles carried into the size press nip may damage the roll coating in the size press. It may also occur that the deposit separated from the blade tip remains on the roll face after the size press nip, and when it enters further under the coating blade, it produces a scratch in the roll face.
With respect to prior art devices, reference is made to the assignee's Finnish Patent Applications Nos FI 901965 and FI 911345 which describe the use of a large-diameter smooth bar is described in connection with a film size press technique, in particular when the film size press is used for pigment coating of a web with high dry solids contents in the paste, with large coating quantities, and with high coating speeds. FI 901965 corresponds to U.S. Pat. No. 5,122,396, the specification of which is hereby incorporate by reference herein. Further, reference is made to the assignee's Finnish Patent Application No. FI 912260 which describes the use of a twin-loaded blade doctor as coating doctors for the roll faces in a film size press. By utilizing the twin-loaded blade doctor, the device prevents cavitation arising from an incorrect geometry of the coating blade. FI 911345 corresponds to U.S. Pat. No. 5,246,497, the specification of which is hereby incorporated by reference herein. FI 912260 corresponds to U.S. Ser. No. 07/878,862 filed May 5, 1992, (assigned to the same assignee herein) now U.S. Pat. No. 5,286,526, the specification of which is hereby incorporated by reference herein.
In the operation of the above mentioned prior art methods and devices, problems have been encountered in that streaks are formed in the coating film in the film size press. In the situation where a coating blade is used, the streak formation is manifested by the coating paste adhering to the tip of the coating blade, and drying at that location, with resulting streak formation in the coating film. On the other hand, with the use of a large-diameter smooth bar, with high coating speeds higher than about 800 m/min and with high dry solids contents of the paste, e.g., higher than about 50% the coating film in the film size press is scratched with gently curved edges. This streak formation is the result of a cavitation effect as the coating film has been separated from the face of the bar at the trailing side of the bar.